A non-aqueous electrolyte secondary battery as typified by a lithium ion battery comprises a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte. The positive electrode comprises a positive electrode current collector and a positive electrode active material layer carried thereon, and the negative electrode comprises a negative electrode current collector and a negative electrode active material layer carried thereon.
A positive electrode is usually prepared by applying, onto a positive electrode current collector, a positive electrode material mixture comprising a positive electrode active material, a conductive agent, a binder, etc, which is then dried, optionally rolled out, and cut into a predetermined shape. Likewise, a negative electrode is usually prepared by applying, onto a negative electrode current collector, a negative electrode material mixture comprising a negative electrode material, a binder, etc, which is then dried, optionally rolled out, and cut into a predetermined shape. The positive and negative electrode material mixtures contain dispersion mediums and are in the form of paste, which are respectively applied onto one face or both faces of the positive and negative current collectors.
It is effective, in the electrode plate, to improve adhesion of the current collector to the active material layer in order to yield a battery with excellent high temperature storage and charge/discharge cycle characteristics. In view of this, Japanese Laid-Open Patent Publication No. Hei 6-260168 proposes to use, as a current collector, a metallic copper with a roughened surface having asperities of 0.1 to 20 μm. Furthermore, Japanese Laid-Open Patent Publication No. Hei 11-310864 proposes a current collector comprising copper whose ratio I200/I220 of an intensity I200 of a peak attributed to (200) plane to an intensity I220 of a peak attributed to (220) plane in an X-ray diffraction pattern is not less than 0.3 and a copper oxide film carried on the surface thereof.
At the same time, portable devices in recent years have been required to be compact and light-weight, and operate for long periods of time; therefore, it is imperative to improve energy density of batteries that serve as a power source for portable devices. In order to cope with the requirements, a thin current collector is essential.
A conventional current collector, however, has a low strength in a thin form while it has slightly improved adhesion to an active material layer. Accordingly, an active material layer exfoliates during the high temperature storage or repetition of charge/discharge of the battery; in some cases, a current collector is ruptured during charge/discharge, furthering capacity degradation.